This invention relates to magnetic recording and/or reproducing apparatus and, more particularly, to such apparatus employing plural magnetic heads which are selectively rendered operative to record and/or reproduce a signal.
Magnetic recording and/or reproducing apparatus employing a plurality of magnetic heads finds ready application in video tape recording (VTR) apparatus. Generally, such recording and/or reproducing apparatus is provided with a guide drum and a plurality of rotary magnetic heads disposed about the periphery of the guide drum. A magnetic recording medium is wound or otherwise deployed about at least a portion of the surface of the guide drum to permit the rotary magnetic heads to scan such recording medium. If the magnetic recording medium is a conventional magnetic tape, the rotary magnetic heads scan successive parallel lines which are skewed with respect to the length of the tape. Depending upon the mode of operation, the rotary heads serve to record signals on or reproduce signals from the scanned parallel tracks. If the magnetic recording medium is a magnetic sheet, the rotary heads function to scan parallel tracks across the surface of such sheet. Relative motion is imparted in an axial direction between the magnetic sheet and the rotary heads such that a multiple of parallel signal tracks are provided. Generally, the rotary heads are spaced such that when one rotates out of recording (or reproducing) relationship another head simultaneously rotates into such recording (or reproducing) relationship. In accordance with one example of typical prior art apparatus, one rotary head leaves the sheet just as another moves into contact therewith. Clearly, signals supplied to both of such rotary heads might be recorded simultaneously by the departing and entering heads. Such signal overlap can result in undesired and deleterious affects and is usually avoided by providing a switching circuit which determines the appropriate rotary head to be supplied with a signal for recording or reproducing. That is, the signal supplied to the departing rotary head will be switched over to the entering rotary head at the proper time so as to eliminate signal overlap.
If the signal supplied to the rotary heads for recording (or reproducing) includes a periodic interval during which no information is present, such as the blanking interval of a television signal, the switching circuit is synchronously operated to effect the change-over during such blanking interval so as to avoid switching noise which might otherwise be present. However, if the signal to be recorded does not include such periodic blanking interval, or if such signal is provided with an irregular or random blanking interval, the synchronous operation of the switching circuit will not provide change-over during periods of no information. For example, if an audio signal is to be recorded by the aforenoted magnetic recording and/or reproducing apparatus, the absence of a periodic blanking interval in the audio signal prevents the timed operation of the switching circuit during periods of no information. Consequently, the inherent switching noise associated with such change-over switching operation will be present. This is an attendant disadvantage of prior art magnetic recording apparatus.